Given that the iconic Lee De Forest’s first ever active electronic amplifying device initiated the 20th Century electronics revolution, will it still work today when used as the output tube of a high fidelity audio amplifier?
By: Ringo Bones
Ever since the 1970s when dedicated Japanese audiophiles
were experimenting with first generation vacuum tubes that were first designed
and manufactured during the 1920s and 1930s into single-ended triode zero negative
feedback audio amplifiers, audio amplifier design technologies long ago
abandoned by the industrial West, there are probably dedicated audio hobbyist
today that are starting to wonder if the Lee De Forest designed iconic first
triode vacuum tube- the Audion – would be feasible when used as an output audio
tube in a single-ended triode audio amplifier design. But will such an unseemly
“old school” electronic audio design still viable and work well into the second
decade of the 21st Century?
Based on established written history on the development of
electronic engineering, it was the pioneering work of Nikola Tesla and Guglielmo
Marconi on radio that initiated the development of the thermionic vacuum tube.
Before the discovery of the germanium detector, early crystal detectors
employed a piece of galena – a type of lead ore – and a catwhisker. Not all
spots on the galena were sensitive and you had to hunt for a spot to touch with
the catwhisker. A slight vibration on the workbench and you’ll lose the sensitive
spot. Also, if the galena should become “dirty” via further exposure to
atmospheric oxygen, you might never find a sensitive spot for your crystal
radio set to work again. Obviously, the galena detector had serious drawbacks.
Oddly enough, the first hint as how to improve the detector
came in 1883, long before the crystal detector was first used as a radio
receiver – or become a favorite of electronic hobbyists and elementary school
level science projects. In that year, Thomas Alva Edison was experimenting with
filaments for his new invention – the electric light bulb. He placed a filament
in a glass bulb and then exhausted the air, creating a vacuum. By means of an
electric current, he tested the filament until it glowed brightly and produced
light.
Edison soon observed an undesirable feature about his bulbs.
After short time, a black substance was deposited on the inside of the glass,
interfering with the light given out. In an attempt to eliminate this deposit
on the glass, Edison inserted a metal plate. Now, this plate did not help much
to solve the problem, but one day he connected a delicate electric meter
between the plate and the positive end of the filament. To Edison’s amazement,
the meter showed that a small electric current was flowing through the circuit.
He did not know why this current should flow and he merely jotted down this
strange fact in his notebook and forgot about it.
Today, we know why this current flows. When a filament is
heated to incandescence (heated to when it becomes hot enough to give off
light), it shoots off streams of electrons. This behavior is known as the
“Edison Effect” or “Thermionic Effect”, of a filament heated to incandescence.
These electrons given off by the hot filament collect on the cool plate and, if
a path is furnished for them, they will flow along the path of the filament.
The electric meter in that path can show that electrons are flowing.
Since the discovery of the Edison Effect back in 1883,
electron theory was still a relatively under-investigated phenomena. But in
1904, J. Ambrose Fleming, an Englishman, who understood the flow of current in
terms of electrons, decided to experiment a bit. To depend upon the electrons
piling up on the cool plate, thought Fleming, is too slow. Suppose we were to create
an actual deficiency of electrons on the plate by placing a positive charge on
it, wouldn’t that attract still more electrons from the filament? Fleming
connected a battery in the circuit from the plate to the filament in such a way
that the positive post of the battery was connected to the plate. He also
connected another battery to the filament to heat it to incandescence. Note
that this filament battery is not in the plate circuit. With such set-up,
Fleming basically invented the first thermionic vacuum tube diode.
Soon after Fleming’s thermionic vacuum tube diode appeared,
in 1907, an American inventor, Lee De Forest, undertook to carry further some
ideas suggested by one of Fleming’s experiments. De Forest knew that when
Fleming placed a positive charge on the plate of his tube by means of a battery
connected between the plate and filament, a much greater electric current
flowed through the meter than when there was no such charge. Further, the
greater the positive charge on the plate, the greater the flow through the
meter. Actually, this did not go on forever, after the positive charge reached
a certain value, placing a greater positive on the plate had no further effect.
It was then that Lee De Forest had a stroke of genius. Since
the flow of current in the plate circuit starts with the stream of electrons
shot out by the heated filament, he began to experiment with that electron
stream. After more experiments, De Forest eventually met the difficulty of
practical amplification of weak RF signals received by the Fleming thermionic
diode detector by making the new electrode in the form of a mesh of very fine
wire – a grid. Since most of the grid consisted of open space, most of the
electrons pulled over by a positive charge on the grid now shoot through these
open spaces an d continued right on the plate. The grid was the solution to his
problem. Since charges on the grid control the flow of electrons from the
filament, we are able to control the large plate currents by means of a small
charge on the grid – and this is what De Forest set out to do and eventually
created the first ever triode thermionic vacuum tube which he called the
Audion. When Dr. Lee de Forest placed a third element, the grid, between the
cathode and the plate of the Fleming thermionic vacuum tube diode, he
introduced the magical word – amplification.
Given that it was from the first improved Edison light bulbs
that the first thermionic vacuum tube triode were derived by Lee de Forest in
1905, every hi-fi hobbyist is probably now wondering whether those old Audion
tubes could be made into single ended triode audio amplifiers. After all,
during the 1990s, tubes developed and manufactured during the 1930s – as in the
Western Electric 300B – or from the 1920s – like the old light bulb looking
PX25 tube – were successfully used in the finest sounding hi-fi audio
amplifiers during the latter half of the 1990s, would an even older triode tube
– as in the turn of the century (early 1900s that is) era Audion even sound
better?
Looking at a typical Audion tube – even recently constructed
working replicas constructed with parts that then exist before 1910, one would
wonder if this tube would be more” microphonic” in comparison to “newer”
designs like the 300B or the PX25. And given that the Audion doesn’t have the
much improved plate current ratings – therefore lower output impedance – of the
output power tubes that were invented years after it – like the PX25 from the
mid 1920s or the Western Electric 300B that cam much later in the 1930s, one
would wonder if the Audion would produce a “good musical sound” if connected in
parallel since most output transformers in the hi-fi DIY market today were more
likely to have been designed to work with the more robust plate current capable
PX25s and 300Bs. But like most DIY audiophiles, I would probably never pass the
chance to hear first hand the sound of a working SET amplifier using authentic
period-correct replica De Forest’s Audion tubes.
